In recent years, there has been rapid progress in reducing the size and weight of mobile information terminals such as mobile phones, laptop computers and personal digital assistants (PDAs), and high capacities are required in the batteries providing the power that drives them. Nonaqueous electrolyte secondary batteries that are charged and discharged by the transfer of lithium ions between the positive and negative electrodes during charge and discharge are widely used as the power sources for driving the mobile information terminals described above because they have a high energy density and high capacity. Furthermore, these batteries have not been limited to mobile applications such as mobile phones, and in recent years their use has been expanded to applications for mid- to large-size batteries in power tools, electric vehicles and hybrid vehicles.
The use of metallic lithium, an alloy that occludes or releases lithium ions, a carbon material or the like for the negative electrode active material and the use of a lithium-transition metal oxide represented by the chemical formula LiMO2 (where M is a transition metal) for the positive electrode material for the nonaqueous electrolyte secondary batteries described above are known. Cyclic carbonates such as ethylene carbonate and propylene carbonate, cyclic esters such as γ-butyrolactone, chain carbonates such as dimethyl carbonate and ethylmethyl carbonate have been used alone or in combination in the electrolytes for these batteries.
Furthermore, lithium cobalt oxide (LiCoO2) is typically illustrated as a lithium-transition metal oxide for the positive electrode material described above and has been used practically as a positive electrode active material for a nonaqueous electrolyte secondary battery. However, when the lithium-transition metal oxide as typified by lithium cobalt oxide having a layered structure is used alone for the positive electrode active material, changes in the volume of the positive electrode active material during charging and discharging reduce the capacity with repeated charging and discharging. In other words, deterioration of charge-discharge cycle characteristics occurs.
Therefore, the following techniques have been proposed.
(1) Inclusion of magnesium in the lithium-transition metal oxide (Japanese Patent Laid-open Publication No. 8-185863).
(2) Inclusion of 10 atomic % or less of at least one metal element selected from zirconium, magnesium, tin, titanium and aluminum in the lithium-transition metal oxide. (Japanese Patent Laid-open Publication No. 2003-45426).
(3) Surface treatment of the lithium-transition metal oxide with a compound represented by ALOk (A being at least one element selected from alkali metals, alkaline-earth metals, Group 13 elements, Group 14 elements, transition metals and rare earth elements, L being an element capable of forming a double bond with oxygen, and k being a number in the range from 2 to 4) and among these one represented by AlPOk (wherein A in ALOk described above is Al and L is P) (Japanese Patent Laid-open Publication No. 2003-7299).
Each of the techniques (1)-(3) improves the cycle characteristics.
However, the techniques (1)-(3) have a problem of reducing the initial efficiency of the positive electrode active material. Therefore, the requirement for increasing the energy density of a nonaqueous electrolyte secondary battery using materials with high initial charge and discharge efficiency (hereinafter sometimes referred to as “initial efficiency”) cannot be satisfied. In recent years, especially, it has been proposed to use active materials having high initial efficiency, such as graphite, as the negative electrode active material to increase the energy density of a nonaqueous electrolyte secondary battery. However, unless the initial efficiency of the positive electrode is improved, the energy density of the battery cannot be increased.